This invention relates to the field of optical devices of the type which record or read information onto or from a storage media along one or more parallel tracks. More particularly, the invention is directed to a system for positioning and focusing an objective lens over a track for the purpose of reading and/or recording.
One type of recording medium that utilizes a large number of tracks is a video disc. To read information from a video disc the video disc is rotated at a relatively high rate of speed and a light source such as a laser beam is focused on one of the tracks of the disc as a light spot. The light spot (beam) is modulated or interrupted as a function of the recorded information and the modulated beam is utilized to reconstruct the recorded information. One important property of such a recording scheme is the ability to have a lare number of tracks positioned closely adjacent to each other on the video disc, resulting in the width of each track and the space between successive tracks being very narrow. The diameter of the light spot, used either for recording or for reading, therefore has to be relatively small. Such being the case, the light spot must be maintained as a focused spot using some form of dynamic focusing. In addition, if the light spot is not maintained in alignment over the track that is desired to be read or recorded upon, an error will be produced.
Therefore, it is also desirable to have a system wherein the light spot may be moved in a direction perpendicular to the track, also in a dynamic fashion, so as to maintain the spot in alignment with the track as the track is rotated on the disc.
Systems for obtaining fine positional control for the focusing and tracking of objective lenses have traditionally utilized interconnected mechanical moving parts involving springs, membranes, coils or other mechanical contrivances.
In U.S. Pat. No. 4,419,614 entitled "Apparatus for Driving Objective Lens in Tracking Direction" by K. Kimura there is disclosed an apparatus for driving an objective lens in a tracking direction perpendicular to the optical axis of the objective lens and to the path of the track. The apparatus is comprised of first and second permanent magnet members arranged symmetrically with respect to a plane which includes the optical axis of the objective lens and which is at right angles to the tracking path direction. First and second magnetic members are arranged across the poles of the first and second permanent magnet members with a third magnetic member arranged within a space surrounded by the first and second permanent magnet members and the first and second magnetic members being movable in the tracking direction together with the objective lens. First and second coils are wound around the first and second magnetic members with the third magnetic member and objective lens being movable in the tracking direction by supplying a tracking control signal to the first and second coils.
A magneto-electro transducer is provided for detecting a magnetic flux leaking from the third magnetic member to produce an electrical signal representing the displacement of the third magnetic member and the objective lens in the tracking direction.
A means is provided for feeding back the displacement electrical signal to the tracking control signal to suppress a resonant vibration of the third magnetic member and the objective lens.
In the aforementioned patented system the third magnetic member and the objective lens are supported movably in the tracking direction by means of a resilient supporting member such as a pair of leaf springs which extend in the direction of the optical axis. A system which could eliminate the interconnected mechanical moving parts, such as the pair of leaf springs would have a higher resonant frequency and would not deteriorate with time.